Modem research and technology have created major changes in the lives of many people. A significant example of this is optical communication. Over the last two decades, fiber optic lines have taken over and transformed the long distance telephone industry. Fiber optic lines also play a dominant role in making the Internet available around the world. When fiber optic lines replace copper wires for long distance calls and Internet traffic, costs are dramatically lowered and the rate at which information can be conveyed is increased.
To maximize information bandwidth, that is, the rate at which information can be transmitted, it is generally preferable for multiple information signals to be conveyed over the same optical fiber, with each information signal being conveyed as an optical signal having a different frequency. This technique is called wavelength division multiplexing (WDM).
A leading example of WDM is the standardized frequency bands for optical communication that have been established by the International Telecommunication Union (ITU), such as the “C” or conventional communication band, the “L” or long wavelength communication band, and the “S” or short wavelength communication band. The “C” communication band is widely used and includes channels that range in frequency between 191.5 and 196.25 terahertz (THz). The “L” communication band is currently a leading edge technology and includes channels between 186.5 and 192.85 THz, while the “S” communication band may become important in the future.
The “C” communication band is divided into about 24 channels that are separated in frequency by increments of about 0.2 THz, which is 200 gigahertz (GHz). If the optical communication system can support channels with frequencies that differ by an increment of only about 50 GHz, then this range can support about 96 channels.
In optical communication systems, it is sometimes necessary to shift a particular optical signal from one channel to another. Reasons for changing the channel of an optical signal include, for example, the following:                Routing considerations: for example, conveying an optical signal on a first fiber optic line from Boston to New York City using a first channel and then conveying the same signal on a second fiber optic line from New York City to San Francisco using a second channel because, for example, the channel is occupied on the second line;        Transitioning from a first optical communication system that supports one set of channels to a second system that supports a different set of channels, for example, from the “C” communication band to the “L” communication band;        Compensating for failures in the communication system equipment; or        Transitioning between optical communication systems that operate with different frequency increments between channels.        
Currently, a common method for shifting the frequency of an optical signal requires the regeneration of the optical signal. For example, the frequency shift of an optical signal may be accomplished by first converting the optical signal into an electronic signal, and then re-converting the electronic signal into another optical signal having the desired frequency. This conversion from optical to electrical and then electrical to optical requires knowing certain characteristics of the modulation of an optical carrier such as the particular bit format and frequency range of the modulation. This can be overly restrictive in some applications. Although frequency shifting using reconverted optical signals may be useful in many applications, these systems are expensive to operate and maintain.